Fix #84 - Created rule G-7730: Avoid multiple DML events per trigger if primary key is assigned in trigger.

Author: kibeha

docs/4-language-usage/7-stored-objects/7-triggers/g-7730.md

@@ -0,0 +1,62 @@
+# G-7730: Avoid multiple DML events per trigger if primary key is assigned in trigger.
+
+!!! warning "Major"
+    Efficiency, Reliability
+
+## Reason
+
+If a trigger makes assignment to the primary key anywhere in the trigger code, that causes the session firing the trigger to take a lock on any child tables with a foreign key to this primary key. Even if the assignment is in for example an `if inserting` block and the trigger is fired by an `update` statement, such locks still happen unnecessarily. The issue is avoided by having one trigger for the insert containing the primary key assignment, and another trigger for the update. Or even better by handling the insert assignment as ´default on null´ clauses, so that only an `on update` trigger is needed.
+
+## Example (bad)
+
+``` sql
+create or replace trigger dept_br_iu
+before insert or update
+on departments for each row
+begin
+   if inserting then
+      :new.department_id := department_seq.nextval;
+      :new.created_date  := sysdate;
+   end if;
+   if updating then
+      :new.changed_date  := sysdate;
+   end if;
+end;
+/
+```
+
+## Example (better)
+
+``` sql
+create or replace trigger dept_br_i
+before insert
+on departments for each row
+begin
+   :new.department_id := department_seq.nextval;
+   :new.created_date  := sysdate;
+end;
+/
+
+create or replace trigger dept_br_u
+before update
+on departments for each row
+begin
+   :new.changed_date  := sysdate;
+end;
+/
+```
+
+## Example (good)
+
+``` sql
+alter table department modify department_id default on null department_seq.nextval;
+alter table department modify created_date  default on null sysdate;
+
+create or replace trigger dept_br_u
+before update
+on departments for each row
+begin
+   :new.changed_date  := sysdate;
+end;
+/
+```